Structure-Function Analysis of Vaccinia Virus H7 Protein Reveals a Novel Phosphoinositide Binding Fold Essential for Poxvirus Replication

Abstract

Phosphoinositides and phosphoinositide binding proteins play a critical role in membrane and protein trafficking in eukaryotes. Their critical role in replication of cytoplasmic viruses has just begun to be understood. Poxviruses, a family of large cytoplasmic DNA viruses, rely on the intracellular membranes to develop their envelope, and poxvirus morphogenesis requires enzymes from the cellular phosphoinositide metabolic pathway. However, the role of phosphoinositides in poxvirus replication remains unclear, and no poxvirus proteins show any homology to eukaryotic phosphoinositide binding domains. Recently, a group of poxvirus proteins, termed viral membrane assembly proteins (VMAPs), were identified as essential for poxvirus membrane biogenesis. A key component of VMAPs is the H7 protein. Here we report the crystal structure of the H7 protein from vaccinia virus. The H7 structure displays a novel fold comprised of seven ␣-helices and a highly curved three-stranded antiparallel ␤-sheet. We identified a phosphoinositide binding site in H7, comprised of basic residues on a surface patch and the flexible C-terminal tail. These residues were found to be essential for viral replication and for binding of H7 to phosphatidylinositol-3-phosphate (PI3P) and phosphatidylinositol-4-phosphate (PI4P). Our studies suggest that phosphoinositide binding by H7 plays an essential role in poxvirus membrane biogenesis. IMPORTANCEPoxvirus viral membrane assembly proteins (VMAPs) were recently shown to be essential for poxvirus membrane biogenesis. One of the key components of VMAPs is the H7 protein. However, no known structural motifs could be identified from its sequence, and there are no homologs of H7 outside the poxvirus family to suggest a biochemical function. We have determined the crystal structure of the vaccinia virus (VACV) H7 protein. The structure displays a novel fold with a distinct and positively charged surface. Our data demonstrate that H7 binds phosphatidylinositol-3-phosphate and phosphatidylinositol-4-phosphate and that the basic surface patch is indeed required for phosphoinositide binding. In addition, mutation of positively charged residues required for lipid binding disrupted VACV replication. Phosphoinositides and phosphoinositide binding proteins play critical roles in membrane and protein trafficking in eukaryotes. Our study demonstrates that VACV H7 displays a novel fold for phosphoinositide binding, which is essential for poxvirus replication. P hosphoinositides are lipids derived from reversible phosphorylation of phosphatidylinositol at positions 3, 4, and 5 of the inositol head group (1). They include three monophosphorylated (phosphatidylinositol-3-phosphate [PI3P], PI4P, and PI5P), three bisphosphorylated [PI(3,4)P 2 , PI(3,5)P 2 , and PI(4,5)P 2 ], and one trisphosphorylated [PI(3,4,5)P 3 ] isoform. Cellular organelles are partially defined by their phosphoinositide compositions, and specific recognition of phosphoinositides on different organelles is crucial for protein sorting and memb...

Poxvirus membrane biogenesis

“…H7 is not detected in highly purified MVs and is not strongly retained in viral factories following synthesis (Satheshkumar et al, 2009). The crystal structure of H7 reveals a novel fold comprised of seven α-helices and a highly curved three-stranded antiparallel β-sheet (Kolli et al, 2014). A basic patch representing a phosphoinositide-binding site is essential for binding to phosphatidylinositol-3- and 4-phosphate in vitro (Kolli et al, 2014).…”

Structural basis for antagonizing a host restriction factor by C7 family of poxvirus host-range proteins

“…After washing with lysis buffer, the beads were resuspended in SDS sample buffer, and the eluted proteins were resolved by SDS/PAGE and detected with Western blot as described previously (35). The detection antibodies were mouse monoclonal antibodies against V5 (clone V5-10; Sigma-Aldrich) and VACV E3 (36) and rabbit anti-SAMD9 polyclonal antibody (HPA-21319; Sigma-Aldrich).…”

The membrane binding and deformation property of vaccinia virus K1 ankyrin repeat domain protein

“…Among the 27 vaccinia virus proteins, H7 and A6 proteins are known to function in virion membrane biogenesis. The H7 and A6 proteins bind to phosphoinositides and to glycerophospholipids, respectively (Kolli et al, ; Pathak et al, ). From our assay, we found that H7 (ORF 41) and A6 (ORF 204) precipitated with the Folch fraction liposomes (Figure b,e), suggesting the suitability of our screening for identification of soluble membrane‐binding proteins.…”